1. Field of the Invention
The invention concerns a circulator suitable for use in an external magnetic field, in particular of a magnetic resonance apparatus, of the type having a flat ferrite structure.
2. Description of the Prior Art
To excite nuclei in magnetic resonance and to acquire the response signals, magnetic resonance apparatuses employ radio-frequency antennas, in particular whole-body antennas that are arranged inside the magnet and must be activated with correspondingly high powers in the kilowatt range to emit the excitation signal. At least one amplifier is therefore provided that feeds a signal to the input of the radio-frequency antenna. Instead of a single amplifier for the entire antenna, it has been proposed to use multiple amplifiers with output signals that exhibit adjustable phase differences relative to one another, in order to respectively feed separate antennas. The intensity distribution of the transmission field within the excitation volume should thus be better adapted to the requirement of the magnetic resonance. Ideally, instead of one amplifier that delivers the total power, eight individual amplifiers are then required from which only one eighth of the total transmission power must be delivered.
So that the amplifier can operate ideally, the connected load impedance must always correspond to 50 Ohm. Depending on the weight and volume of the patient to be examined, however, the antenna impedance changes, and thus the load impedance for the amplifier also changes. However, a pre-adaptation can only be implemented for a standard load case, such that in most cases a portion of the power sent to the antenna is reflected at the feed point and arrives back at the amplifier. In order to solve this problem, the following possibilities are known.
First, the reflected power can simply be allowed to transduce into heat in the amplifier, which leads to an over-dimensioning of the cooling requirement and the structural size. This is reflected in high costs of the amplifier. Additionally, the risk exists that the current and voltage peaks of the reflection power can destroy the end stage of the amplifier.
Another possibility, in which the amplifier is protected from reflected power, is to use what is known as an adaptation tuner that minimizes the reflected power for every load case before the actual measurement. In particular given use of multiple amplifiers, such a procedure has disadvantages. A much greater circuit complexity is required for the adaptation tuner and the time cost for the load compensation is greater.
Therefore it has been proposed to interpose a circulator between the amplifier and the terminal of the magnetic resonance antenna, which circulator relays the transmission power arriving from the amplifier nearly without loss to the magnetic resonance antenna while the reflected power arrives at the third output of the circulator and there is consumed in a power dump (thus a reflection-free power termination) or is converted into heat.
A circulator is a non-reciprocal radio-frequency component. In the present case a circulator with three terminals is used. An ideal circulator ensures that a signal is relayed from one terminal to another in one direction practically without attenuation and free of reflection. For example, a signal can be relayed only from terminal 1 to terminal 2, from terminal 2 to terminal 3 or from terminal 3 to terminal 1. In order to achieve this non-reciprocal transmission, microwave ferrites are used that are saturated by a strong magnetic field. The ferrite structure used (also called a resonator) is essentially formed by two generally cylindrical ferrite plates between which a conductor trace structure is enclosed. The required constant magnetic field is generated by permanent magnets arranged above and below the ferrite structure. A housing or cover often serves as a yoke to close the magnetic circuit. Magnetic field lines are in principle closed. To generate a predictable magnetic field it is typical to merge the magnetic field into a magnetic circuit through components that are particularly good conductors in order to optimally avoid scatter field losses. Via such field conductor elements, the magnetic flux can be directed in specific, desired paths.
In order to obtain an optimally ideal circulator, it is important to find the correct operating point, thus in particular to find an optimal operating field. For real circulators it is additionally known that a temperature dependency exists. Known circulators accordingly require a static magnetic operating field of specific size which is produced by permanent magnets.
In magnetic resonance apparatuses or magnetic resonance antenna devices, cable damping (i.e. losses due to cables that are too long) should be optimally minimized, such that the amplifier should be arranged in proximity to the antenna. The power demand at the amplifier is thereby distinctly lowered. For the possibility of a decoupling of the reflected power via circulators, this means that these must likewise be arranged in the region of the antenna, but when the circulators are placed at that location, the strong magnetic scatter field of the magnetic resonance apparatus alters the field that is present in the region of the ferrite structure away from the optimal operating field such that the circulator loses its function. The magnetic properties of the permanent magnets additionally suffer a lasting change due to the strong scatter field of the magnetic resonance apparatus. Placement in proximity to the magnetic resonance apparatus is not reasonable in this case.
To solve this problem it has been proposed to use a circulator without permanent magnets that utilizes the fundamental “interfering” scatter field of the magnetic resonance apparatus. For this, the circulator would have to be mounted at suitable positions in the scatter field at which orientation and size of the magnetic field coincide with the optimal operating field as was generated by the previously present, but now removed, permanent magnets. However, ultimately this solution would be practical only in rare cases since design limitations (for example the attachment, the cooling or the wiring) to arrange the circulator exactly at the matching points in the scatter field are not allowed. An additional problem is that the scatter field is not constant over time, and disruptions thus can also occur.